Signatures of a ferro-Josephson effect in twisted graphene

In the world of modern electronics, scientists are constantly searching for materials that can carry information in new ways, often by manipulating the tiny magnetic spins of electrons. For decades, a major focus has been on how these spins behave when they encounter boundaries between regions of different magnetic alignment, known as domain walls. In standard magnetic materials, pushing a current through such a wall usually just causes the wall to slide or get stuck, a process driven by friction-like forces. However, a more exotic possibility exists where the flow of electricity does not just move the wall, but causes the magnetic moments within it to wobble or spin in a rhythmic fashion. This rhythmic motion, if it occurs, could generate a voltage in a way that is fundamentally different from ordinary resistance, creating a signal that is tied to the speed of the spin's rotation rather than the strength of the current. Understanding whether this phenomenon can be observed and controlled in new materials is crucial for developing future technologies that rely on the precise manipulation of electron spins.

A team of researchers has now found strong evidence for this effect in a specialized device made from twisted layers of graphene. By stacking a single layer of graphene on top of three layers of the same material and twisting them slightly relative to one another, they created a structure where electrons behave in a highly coordinated, collective manner. When they cooled this device to extremely low temperatures and applied specific electrical settings, they discovered that the material spontaneously developed regions with different magnetic orientations. The most striking discovery occurred when they applied a tiny magnetic field parallel to the surface of the material. At exactly zero field, the electrical resistance of the device spiked dramatically, but only when a small current was flowing. As soon as the current was turned off, the spike vanished. This behavior suggested that the current itself was driving the magnetic boundaries to spin, creating a dynamic state that resisted the flow of electricity.

The researchers observed that this resistance peak was incredibly sensitive to the direction and strength of the magnetic field. Even a field as weak as a few hundred microtesla was enough to suppress the effect, forcing the magnetic boundaries to lock into a fixed position until the current became strong enough to overcome that lock. This threshold current grew linearly with the magnetic field, a relationship that matched a theoretical prediction for a phenomenon the authors call the ferro-Josephson effect. In this scenario, the current acts like a torque, trying to twist the magnetic moments within the wall. When the magnetic field is zero, the moments are free to spin, and this spinning motion generates a voltage drop that looks like a sharp peak in resistance. When a small magnetic field is applied, it acts as a brake, holding the moments in place until the current is strong enough to break that hold and restart the spinning. The researchers confirmed that this effect is not caused by the current simply heating the material or pushing the magnetic walls past physical obstacles, as those mechanisms would not show such a precise dependence on the magnetic field or disappear so cleanly when the current is removed.

To prove that the effect relied on the specific arrangement of magnetic domains, the team performed experiments where they deliberately reset the magnetic state of the material. They found that the resistance spike would only appear if the material contained a specific mix of magnetic regions: one region where the spins were locked firmly in a vertical direction by the material's internal structure, and a neighboring region where the spins were free to rotate easily in the horizontal plane. When the researchers used an external magnetic field to align all the spins in the same direction, the resistance spike disappeared. However, as soon as they swept the field back through zero, the easy-to-rotate region would flip its direction while the locked region stayed put, recreating the boundary needed for the effect to return. This behavior confirmed that the phenomenon was a direct result of the interaction between the flowing current and the boundary between these two distinct magnetic states.

The study highlights that this effect operates at energy scales far smaller than the thermal energy present even at the lowest experimental temperatures, suggesting that it is a purely quantum mechanical process driven by the collective motion of electrons. The researchers note that while their explanation fits the data perfectly, the exact details of how the electrons interact with the spinning wall are still being refined. They propose that the effect could serve as a highly sensitive tool for probing the invisible magnetic textures inside these materials, offering a new way to study how electrons organize themselves in complex quantum systems. By demonstrating that a simple electrical current can induce a rhythmic, spinning motion in magnetic boundaries, the work opens a new chapter in understanding how to control and utilize these subtle magnetic dynamics in future electronic devices.